fix(plugin-handlers): normalize default agent runtime lookup

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/plugin-handlers/agent-config-handler.test.ts

@@ -191,6 +191,56 @@ describe("applyAgentConfig builtin override protection", () => {
     }
   })
 
+  test("normalizes display-name default_agent to runtime agent name", async () => {
+    // given
+    const config = createBaseConfig()
+    config.default_agent = "Sisyphus - Ultraworker"
+
+    // when
+    await applyAgentConfig({
+      config,
+      pluginConfig: createPluginConfig(),
+      ctx: { directory: "/tmp" },
+      pluginComponents: createPluginComponents(),
+    })
+
+    // then
+    expect(config.default_agent).toBe(getAgentRuntimeName("sisyphus"))
+  })
+
+  test("keeps config-key default_agent behavior unchanged", async () => {
+    // given
+    const config = createBaseConfig()
+    config.default_agent = "sisyphus"
+
+    // when
+    await applyAgentConfig({
+      config,
+      pluginConfig: createPluginConfig(),
+      ctx: { directory: "/tmp" },
+      pluginComponents: createPluginComponents(),
+    })
+
+    // then
+    expect(config.default_agent).toBe(getAgentRuntimeName("sisyphus"))
+  })
+
+  test("keeps fallback default_agent behavior unchanged", async () => {
+    // given
+    const config = createBaseConfig()
+
+    // when
+    await applyAgentConfig({
+      config,
+      pluginConfig: createPluginConfig(),
+      ctx: { directory: "/tmp" },
+      pluginComponents: createPluginComponents(),
+    })
+
+    // then
+    expect(config.default_agent).toBe(getAgentRuntimeName("sisyphus"))
+  })
+
   test("filters user agents whose key matches the builtin display-name alias", async () => {
     // given
     loadUserAgentsSpy.mockReturnValue({

src/plugin-handlers/agent-config-handler.ts

@@ -2,7 +2,11 @@ import { createBuiltinAgents } from "../agents";
 import { createSisyphusJuniorAgentWithOverrides } from "../agents/sisyphus-junior";
 import type { OhMyOpenCodeConfig } from "../config";
 import { isTaskSystemEnabled, log, migrateAgentConfig } from "../shared";
-import { getAgentRuntimeName } from "../shared/agent-display-names";
+import {
+  getAgentConfigKey,
+  getAgentRuntimeName,
+  normalizeAgentForPromptKey,
+} from "../shared/agent-display-names";
 import { AGENT_NAME_MAP } from "../shared/migration";
 import { registerAgentName } from "../features/claude-code-session-state";
 import {
@@ -189,8 +193,10 @@ export async function applyAgentConfig(params: {
 
   if (isSisyphusEnabled && builtinAgents.sisyphus) {
     if (configuredDefaultAgent) {
+      const configKey = getAgentConfigKey(configuredDefaultAgent);
+      const runtimeConfigKey = normalizeAgentForPromptKey(configuredDefaultAgent) ?? configKey;
       (params.config as { default_agent?: string }).default_agent =
-        getAgentRuntimeName(configuredDefaultAgent);
+        getAgentRuntimeName(runtimeConfigKey);
     } else {
       (params.config as { default_agent?: string }).default_agent =
         getAgentRuntimeName("sisyphus");